There is a fascinating PBS Space Time video[1] about black holes that questions the very existence of whatever is "inside" a black hole.
They point out that as you approach a black hole, your proper time is advancing slower and slower when compared to an observer located far away from the black hole (like us, on Earth). That means that from the point of view of this far observer, nothing happens inside a black hole until an infinite amount of time. In other words, what is inside a black hole is separated from us not by a space-like frontier, but a time-like one. Events behind the event horizon are postponed to "the end of times".
Does it even make sense to say that those events exist, then? Are they any more real than fairy tales or mathematical equations? If not, we can make any speculation we want, including the existence of alien civilizations.
Here's something I've wondered about black holes, related to what's inside. Simplistically, there must be mass inside, i.e., every black hole has a mass that can somehow be measured, e.g., by orbital velocities of satellites. And the mass-energy has to equal the mass-energy that has fallen in. So far so good.
But the stuff that falls in is not purely described by its mass. It also has angular momentum, electrical charge, maybe a few other quantities such as charm and strangeness (speculating here). So the black hole must have a net angular momentum, electrical charge, charm, and strangeness. Electrical charge could be measured by holding an electrometer next to the black hole. And so forth.
So far I've described 5 quantities that "describe" a black hole. So my question is: How many more are there? What's the total number of numbers that accompany a particle as it gets sucked in?
Suppose it's a small number, like 11. Does that place an upper limit on the information content of a black hole, and hence the possibility of describing a "civilization" inside it?
I never thought of that in the context of strangeness, lepton number, and other conserved quantities. I don't think strangeness is really measurable (and I don't remember if it's technically conserved), but black holes should probably preserve lepton number somehow. Is that a quantum gravity sort of problem?
It might be, in that a Q-ball or something similar could there instead of a singularity, or it could be that there is no interior. Remember that the Holographic Principle implies that the total entropy of the volume of the hole can be encoded in the 2D event horizon of the hole.
The singularity is a prediction of GTR, but GTR breaks down there too. This is why black holes are so exciting in the context of new theories which are complementary to GTR!
I've often wondered why that 2D event horizon is necessary. The space near the event horizon also gets stretched, from the perspective of things falling towards it. That is, if you're falling towards a black hole, the event horizon is a point that's getting closer, but it's infinitely far away (collision with event horizon will happen at a time in the future further than any other event in the future). Sufficiently stretched space would provide more than enough space to contain all the objects falling in and make it look from the perspective of someone falling in like nothing at all is happening.
It even preserves relative motion ! If A and B are both falling towards the event horizon, their relative movement doesn't change : by that I mean that if it was 2 moons, and someone launched a rocket from A towards B, it would take, say 10 hours to get to B. When both fall towards the black hole that time can still lengthen from the perspective of someone standing on A or B. It can shorten. It can stay the same. Depends on how it was changing before they started falling in.
So it could simply be that relativity stretches the space time near the event horizon enough for everything to fit in there, like that tent in Harry Potter. It looks weird from the outside, but if you're falling into the black hole it's the opposite: everything outside of that (very, very large) space near the event horizon is what looks weird. But if the black hole is big enough, it looks weird, but ... not very much.
The entirely weird thing is, you can choose initial conditions where all distances lengthen proportionally, in fact that's the more common scenario. So for all objects that are falling into the black hole, and objects not falling into it, all distances lengthen. Make the black hole big enough and the difference between things falling into it and things orbiting it or even moving away are very very small indeed.
And now you look at our universe, and that's exactly what you see. The value of the cosmological constant (ie. the universe is expanding, but ever slower and it will never actually stop expanding) can be explained by the assumption that we're falling into a very, very large black hole.
The main issue is that you cannot just put a voltmeter or other devices/methods near a BH. Not that you can't do it in a Gedankenexperiment way, of course, but in a relativistic way. Because of General & Special Relativity, things like volts and amps, charge and mass, they get 'bent' in this weird non-intuitive way. In our own (slow) world, electro-magnetism is one of those 'weird things' that comes out of special relativity [0] for example. So, trying to take our 'classical' world and apply it to a General Relativistic world is going to have 'weird' results.
There is a TON of work done on BHs and the formulations to try to bend our knowledge of QM and 'classical' physics to them. If you really want to get into the nitty gritty of it start with this [1]. It's a HARD road, but the proofs and knowledge in the field is mindblowingly cool. Go for it!
Thanks. I actually got started in Meisner, Thorne, and Wheeler, and it's a slog. I've asked some physicists if they know of an easier textbook, but there doesn't seem to be one. I'm a physicist too, but my willingness to build outrageous experiments made up for my lack of theoretical agility. Still, I'd love to understand gravitation.
I've still got my copy of Jackson from grad school. ;-)
There are lots of easier textbooks, all of them less...wordy. Wald's is the industry standard these days. Schutz, Carroll, and D'Inverno are all good introductions. If you're really pressed for time, Dirac wrote an intro to GR which is about as short as humanly possible. It omits most of the geometry though.
+1 for Wald - MTW is great, but it's really dense, and takes some odd approaches that are not really common in the field. Wald stays lean and tight, while still covering the important stuff.
You'll probably find MTW a lot more approachable after working through a few less beastly texts.
I'm working through Sean Carroll's "Spacetime and Geometry". So far it's been quite accessible and is replete with exercises. Carroll's explanations do a really good job of motivating definitions while also introducing deeper, beautiful math. It's not all just tensor munging.
That's exactly the right number to describe the macroscopic state of a no-hair black hole (defined as the horizon as seen from the outside) using a set of spatial coordinates: mass, charge, angular momentum for three spacelike axes, linear momentum for three spacelike axes, and centre-of-mass position for three spacelike points. Here I have already done a foliation of spacetime into a spatial slice with a constant timelike coordinate; it'd be normal to use coordinates in which the centre of mass is always at the origin, which fixes the last six components at zero.
On the other hand, if we omit the spacetime foliation, we are obliged to use tensor quantities for these variables; moreover, if the spacetime region we consider is not "sufficiently small", then no-hair looks much more conjectural. Foliating this region, we would expect at least the charge no-hair number not to change from one slice to another. For a black hole with a highly ionized accretion disk with nuclei or electrons liable to cross the event horizon at any time, this is a tough ask (and the subject of research in numerical relativity).
> charm and strangeness
The larger the black hole, the flatter the spacetime just outside the event horizon; astrophysical black holes' immediate neighbourhood is too flat to break quark confinement, so a surplus of colour charge seems unlikely. Conversely, a black hole small enough that tidal effects are very strong just outside the horizon is likely to be evaporating so violently that gravitational effects on hadrons just outside the horizon seems (to me, anyway) less interesting than scattering interactions.
> What's the total number of numbers that accompany a particle as it gets sucked in?
Good question. This is an area of research.
Any answer raises a second question: how does "balding" work?
Even fully classically we have this problem: if we drop a thin uniform spherical shell of neutral matter of mass M into a Schwarzschild black hole (so there's no rotation or charge or quantum-anything to consider) how do we distinguish that black hole from an identical setup except we drop in two concentric shells of 1/2 M each?
If in some future region of spacetime when the shells are inside the BH (black hole) we can distinguish between BH with one shell vs BH with two shells, then no-hair is wrong. If we cannot distinguish, then classical information is lost.
This is black hole thermodynamics because we have a relationship between macrostates (the no-hair values) and microstates (the set of values that migrated from outside the BH to inside the BH), and we can define entropy in a Boltzmannian way using that relationship. If no-hair is accurate such that we can throw in an huge number of shells each with a tiny fraction of the mass of our single-shell example, then that black hole's entropy is enormous.
The quantum picture is in some ways "just" a complication of this fully classical information loss problem. If we can throw in whole molecules / bits-of-dust, atoms and ions of various masses, free electrons, photons, neutrinos, and so forth and still see a "no-hair" set of macroscopic variables, then a reasonable-size BH's entropy is enormous, and it gets much more enormous if we only increase the BH mass while keeping the other no-hair values always zero (and letting the others vary does not help much).
If in our universe we find a black hole which we can comfortably describe with a tiny number of variables (e.g. a no-hair black hole), we should expect that it will have lots of hidden microstates thanks to things having infallen (and indeed, for astrophysical black holes, lots of particles from the stellar remnant). Did these bald away in the past? Or, if black holes evaporate, will these hidden microstates be revealed during that process?
> what's inside
Good question. There's a wide variety of guesses by theoreticians.
As we increase our number of observations of BH-BH, BH-NS, and NS-NS (NS for neutron star) mergers where we get decent gravitational wave signals, we can exclude many possible answers to these problems.
Finally,
> an upper limit on the information content of a black hole
No-hair black holes' event horizons are determined by the no-hair variables alone; their entropy limit is related to the surface area of the horizon.
> a "civilization" inside it
We don't know without a full answer to "what's inside". The "Eldar" idea is that for an extremely massive black hole with an improbably large charge (how does one prevent such an object from drawing in matter of the opposite charge?) there may be stable orbits in an interior region. I think even that is a really big stretch.
hawking radiation can come from any point on the surface that constitutes the event horizon. As the black hole decays, it gives back a lot of information.
That's the thing that never made sense to me about the black holes. The closer you are to the event horizon, the faster the time passes for the universe around you. So reaching the event horizon should take infinite amount of the "outside time", so right before you reach the horizon, you should see the whole future of the universe, including its end, if there's any.
Well, from our point of view all incoming mass gets stuck very close to the event horizon. That's not too surprising considering the amount of information of a black hole is proportional to its surface area. So from a far away observer a black hole is more like a sticky sphere than an hollow ball.
What is important to remember is thst our point of view is not special or “right” in any way. There is a difference between our observation of a process like matter falling into a black hole, and the reality of it. Black holes do “eat” and grow, and that occurs despite the intense time dilation near their surface. The light which returns to us will be redshifted to black, and we won’t see the final event, but it does happen in its own proper time, in finite time. When black holes merge we can detect their gravitational waves, evidence that black holes are not “frozen” in reality.
It is also important to distinguish between the implications of a model black hole formed from the uniform collapse of a perfectly spherical dust cloud, from infinity, in an otherwise empty universe, with no charge or angular momentum, from what happens in nature.
Doesn't it mean that only the space around the black hole sees that "infinite amount of time" pass, and not the whole universe?
So in that case, you wouldn't be seeing the "future of the universe" just the "future" (I guess) of the space around the black hole.
I kind of see blackholes in space as vortexes in a lake. Anything that gets "trapped" in the vortex moves much faster, including the water (space-time) itself. It doesn't impact the rest of the lake, except for the things that are on a collision course with the vortex, and then get swallowed by it (and spit back out?).
I think you've got it backwards. As on object approaches a black hole, an outside observer will see that object's clock slow down and eventually stopping as it hits the event horizon. Conversely, the object falling into the black hole will see the rest of the universe's clocks speed up. It could watch stars (far from the black hole) be born and die. If you take a trip close to a black hole's event horizon and then fly away, you could find yourself in the far future.
> As on object approaches a black hole, an outside observer will see that object's clock slow down and eventually stopping as it hits the event horizon.
So I had the idea that smaller black holes are at the center of the sun, the earth and so on, being the principle source of gravity and the "movement" that we see is just us falling into different black holes at the same time, which are also falling into each other. So micro black holes must be at the center of massive particles too. The world line of a photon on the other hand is just the intersection of two event horizons as they grow, so you get a wave model. And that's why you have entanglement: circles have two intersections, so if your model is two dimensional, you get two entanglements. But you can have vastly more complicated geometries and thus assembles of entangled particles.
I don't know the "standard model" well enough to take the analogy any further, not to mention string theory and all that jazz.
It's really reaching a lot to assume black holes are the sole source of gravity or time. (If you want to distill it really far, I think it's more right to say they counteract time than that they cause or provide it. You go near a black hole, and you age less and have less time to do things between the times of external events not near the black hole.)
>So micro black holes must be at the center of massive particles too.
If an object has a schwarzschild radius smaller than its own radius, then it isn't a black hole. That literally describes all non-black-hole objects with mass. That's just the standard manifestation of gravity.
>The world line of a photon on the other hand is just the intersection of two event horizons as they grow, so you get a wave model. And that's why you have entanglement: circles have two intersections, so if your model is two dimensional, you get two entanglements.
Is there any connection here besides that an event horizon and the sum of all possible paths of a photon in a given amount of time are both spheres?
If there were a micro-black-hole inside of a particle, its event horizon would have to be within the particle, or else the particle would just be indistinguishable from a black hole. The particle wouldn't be like a normal particle with an invisible spherical event horizon surrounding it and affecting its interactions.
Wouldn't it just depend on your definition of the area of the black hole? By your logic, I certainly agree that black holes having mass beyond their event horizon is an impossibility from an observation perspective, but how far outside the event horizon do we include in our definition of a black hole? Far enough out, we could certainly observe them gaining mass.
I think one way for matter to reach the black hole might be that when more matter comes close to the black hole the event horizon expands, so that matter that is already close to the event horizon gets incorporated even without moving. I don't know what happens to the matter that gets inside that way, does it remain stuck, giving an onion-like structure to the black hole, or something else ?
Observers outside a black hole (BH) are free to disagree about the location and shape of the horizon. However, there is no observer which is free to say that there is no horizon [1]. This latter point is why the horizon is a physical feature of the universe containing the BH. The former point is why infallers can cross the horizon according to them, while some outside observers will never actually see the crossing [2].
This is not an analogy as much as an example of an external-vs-internal observer problem. When you close the (opaque, insulated) door of your fridge, observers inside will see the (filament of the incandescent) light significantly dim, and if the door stays closed long enough, will see the light thermalize with rest of the internal volume. Someone standing outside the fridge might not even see the initial dimming; indeed, that observer may only ever see the light as "on" (rather than "heating from cold" or "cooling from hot").
[1] We could talk about naked singularities a bit: this usually means that there is at least one outside-the-black-hole observer for which the shape of the horizon is such that the centre of mass-energy of the BH is outside the horizon, rather than an observer for which there is no horizon at all. However, even these scantily clad BHs don't arise in realistic universes described by General Relativity. Fully naked singularites (where at least one observer exists which does not see any horizon at all) require an alternative theory of gravitation, or conditions extremely unlike those anywhere in our universe.
[2] Consider the observation of a supermassive black hole at the edge of the observable universe. From our view here around Earth, we see a race between a very bright star about to cross the black hole's event horizon and the black hole about to cross our Hubble horizon. Observatory A sees the star vanishing behind the horizon just in time; Observatory B sees the BH cross out of observability before the star vanishes behind the BH horizon. A and B have (very slightly) different Hubble horizons focused on them [3], and also with a (n also slightly) different radial distance to the BH horizon. "B" can never directly see the same coincidence of events that "A" sees; should "B" deny the infalling?
[3] Maybe this is illustrative of observer-centred observables? Glories (an optical phenomenon similar to rainbows) are so observer-specific that you and your handheld camera will have different ones (and each of your eyes will have different ones). As noted in the "From the air" subsection, we can tell what seat a photographer of a glory from a plane must have been sitting in. https://www.atoptics.co.uk/droplets/gloim1.htm
Likewise, we can determine the location in spacetime of an observer of a star-into-black-hole event from that observer's detailed description.
That’s a really interesting point. I wonder if there’s any proper theoretical work done on this possibility.
My immediate, naive instinct is that by crossing this limit, the frame dragging effect, would be extremely powerful, to the point where it might increase the radius where Hawking radiation is formed/emitted and increase the rate of Hawking radiation to avoid passing the limit. A sort of self limiting process to prevent breaking the speed limit of c.
But in the time it took to write this out, I remembered angular momentum and realised that notwithstanding the additional angular momentum of the infalling mass, the conservation of angular momentum would just make the event horizon slower as it expands. Which makes the superluminal event horizon unlikely in my mind.
It was response to deleted comment about "sticking shell" inside of a black hole, to illustrate that such shell will have superluminal speed, thus it cannot exist, because any matter will decay.
There are two frames of reference here, and only from the frame of the external observer does what you say hold true. From the point of view of infalling mass, there is no “freeze-frame” and time proceeds normally. The classic example to illustrate this is a black hole with so much mass that you can fall past the event horizon without experiencing significant tidal forces. You could survive the fall, Andy live for hours inside the hole before tidal forces turned you into subatomic sphagetti.
But from our perspective, the outside observers, why do black holes gain mass? It would take an infinite amount of time from our perspective for any mass to even touch the event horizon.
At this point a few issues arise. The first is that what you’re describing is a feature of the Schwarzschild metric, which applies to a model black hole, which is time-independent and eternal. There is no particular reason to believe that this accurately describes black holes in nature. For example this metric can not describe the merger of two black holes, but we now have observational evidence that this does indeed take place.
The biggest issue, aside from the model, is that time dilation is something which only matters when two observers “compare clocks.” Neither observer alone ever experiences a difference. The crew of a 99.9% lightspeed ship doesn’t experience time dilation... until they return home. It makes no sense to talk about the effects of time dilation from the point of view of a one-way trip to the event horizon.
I've never been able to feel comfortable understanding reference frames. Even considering simpler examples: So what if a probe is launched to catch our solar system's recent cigar-shaped visitor. Assume we catch it and want to bring back a sample of equal mass to the probe. So what determines the kinetic energy required to return this sample to earth? Is the delta relative to that of the probe, to the solar system, or to its origin? What if it is was accelerated to 0.1c relative to its launch site in another galaxy, but is only travelling at 0.001c relative to us?
Common sense dictates that the probe and the sample would require equal fuel to return to earth; but to an observer riding this cigar-rock, why would the universe cut our probe some slack if we changed its kinetic energy rather than the observer?
It’s from the frame of reference of the probe you make fuel calculations. The various observers don’t necessarily need to agree on the ordering of events (Relativity of Simultaneity), but they will always agree on the laws of physics, which are the same everywhere. If it takes a given amount of fuel to accelerate a mass to a given degree, everyone will agree on that point. It might take som calculation to make that clear to all of the observers, but they will agree regardless if some are in accelerating reference frames, and others inertial.
Seeing the universe around them is comparing clocks with another frame of reference. On the ship, life at 99.9% and life at 1% of c is identical. By the same token, if you fell into an (inactive) supermassive black hole, your watch would trick the same way into and past the event horizon. You would in fact live inside the hole for hours until you were torn asunder.
The point is, the crew onboard the ship can compare clocks with those in different frames of reference at any time. All they have to do is observe the period of a pulsar for example, or measure the orbits of binary star systems, etc. It would be quite apparent to them that there is a time dilation effect for them with respect to most of the rest of the universe. See Tau Zero, by Poul Anderson. It has a few mistakes, but it's still a great read.
I’m not claiming that they can’t recognize that their relative velocity is much greater than their surroundings. You can even calculate the degree of time dilation you’re experiencing relative to another observer, but I’m not talking about that either.
The biggest issue, aside from the model, is that time dilation is something which only matters when two observers “compare clocks.” Neither observer alone ever experiences a difference. The crew of a 99.9% lightspeed ship doesn’t experience time dilation... until they return home. It makes no sense to talk about the effects of time dilation from the point of view of a one-way trip to the event horizon.
That has to do with the experience of their frame of reference. Time does appear to “slow down” for them, rather everything else will seem to “speed up.” You can infer the difference, but you can’t sctually communicate that or compare with anyone else until you decelerate. In the extreme case of a gravitational event horizon, there will be no ability to ever communicate again. The fact that external observers will see you infinitely redshifted doesn’t imply anything about your experience of subjectively falling past the horizon. Both are valid frames of reference, but ultimately will develop spacelike separation which prohibits further communication.
As it relates to the issue st hand, you can’t make accurate statements about mass never passing through the EH based on observations from a distant from of reference.
> Seeing the universe around them is comparing clocks with another frame of reference
So, if you don't sense anything, you don't sense time dilation either?
This is slightly more complicated. First of all, you haven't given a frame of reference. If you claim someone were moving at 0.99c then you have already set the frame of reference. And they would have to gain near infinite mass and would die. You seem to assume a restricted frame of reference though, inside the spaceship. So, a point of reference inside the spaceship would see light moving with c inside the spaceship. And would assume his own point of reference as the origin of the inertial frame of reference. So baring any outside measurement, how do you know the spaceship is moving with 0.99c and in which frame of reference?
I just wanted to mention that the Schwartzchild metric doesn't only apply to black holes. It's the general metric for a spherically symmetric, static space-time, so it's good for roughly spherical bodies like stars and planets over small timescales.
Recently, I worked out the ISS orbit using the Schwartzchild metric as an approximation of Earth. It's cool to see the orbital period pop out and agree with real life! It's then just a small step to calculate the time dilation experienced by ISS astronauts.
First you have to define mass. It's currently "defined" extrinsically, by a piece of metal machined by the SI. That doesn't allow an intrinsic answer definition of a black holes mass.
The answer is kinda easy if I can make up my own intrinsic definition. The mass is the mass of the stuff around the black hole. A black hole is a singular point, it can't have mass, don't be silly.
off-topic a bit: does anyone have suggestions for paths for the layperson toward being able to reason and intuit about this stuff at one level beyond pop physics claptrap?
I have heard "the theoretical minimum" suggested in the past, but I haven't heard from anyone who has actually used it to go from zero to say, general relativity, only people who think it sounds good. It also seems like a steep investment -- I just want to have a decent enough grasp to separate sense from nonsense and hold a picture in my mind.
One related thing I have wished existed was a good place to ask questions about and discuss topics like this while learning them, but it seems like in theoretical physics, everyone (including myself) has an infinite number of different stupid questions and misconceptions, to the point that we drown each other out, and answers to our questions often have little cross-applicability.
You say you haven't heard from anyone who has used it to go from zero to say, general relativity. You might want to look into the adult alumni of the class.
EDIT: You might also find another HN post interesting, it's currently at the top of HN classic: "Learning classical mechanics through Haskell", here: https://news.ycombinator.com/item?id=16453192
Feynman's lectures were delivered to an audience of graduates and faculty who had signed up for an introductory course because Feynman was teaching it. Reading the Feynman lectures without the math background will end up confusing you horribly - I know this because I've found them not-exactly-light reading even though I already (supposedly) know what they're about!
The introductory physics course for which Feynman lectured in 1961-63 is part of Caltech's core curriculum, required of all undergraduate students. It was taken by the entire freshman class in 1961-62, and the same people, then sophomores, in 1962-63. The course consists of 3 parts: lectures, recitation sections, and labs. The recitation sections and labs are mandatory, but the lectures are not, and many Caltech students - particularly those who do not understand English well - choose not to attend all the lectures. Attrition in Feynman's lectures was typical for this course: an average of about 10%-20% of the students did not attend any given lecture. However, I would like to inform the poster that graduate students and faculty do not "sign up" for this course - that is not possible. Some graduate students and a few faculty members sometimes attended Feynman's lectures, almost all of whom were leading recitation sections or working in the course's labs. The ratio of undergraduate students to others who attended Feynman's lectures was very large, as you can see for yourself in this photo taken during a typical lecture (the ones with ties are faculty): http://www.feynmanlectures.info/FLP%20Classroom.JPG . For more information please see http://www.feynmanlectures.info/popular_misconceptions_about... . Michael A. Gottlieb, Editor, The Feynman Lectures on Physics New Millennium Edition
The only well-traveled path to understanding physics starts at "highschooler with a lot of confidence in their math skills," and it takes four years minimum. Anything else is going to be a long shot, just by virtue of the fact that nobody's doing it (for the most part).
Nobody has ever discovered a way to truthfully map physics concepts into English sentences that you can understand without having to have explored the entire definition tree.
That's because the mappings always fail - not so much a concurrent thing as a consequence. (For comparison, you can say everything about Roman troop organization in English, because that's native to how the information exists in the first place. You won't get any "well, actually" about the size of a legion, because it involves words and concepts that everyone already knows.)
Start with YouTube -- "Crash Course Astronomy" and then "PBS space time". Both programs are very well done, and give fairly rigorous yet entertaining overviews to cosmology and the related physics (PBS space time is the more rigorous one). Crash course is great because it touches on almost all major cosmological phenomena. For some of the videos, you may want to rewatch certain parts for understanding, or supplement with some brief research via google.
This route provides a gentle, productive ramp as you make sure you really want to dive deep.
I've done that and, ideally, I'd like to now buy some college physics books. However, I may have to delay this next step for quite a while due to some other higher priority goals... We will see if I get back to it. There is too much interesting shit and important things to do in the world, heh.
It's a little bit like The Little Schemer in that it often asks you a question and expects you to try to answer it before turning the page (or turning it upside down). The problems tend to go like "if you did this, would that (A) increase (B) decrease (C) stay the same" or "(A) increase by one (B) double (C) quadruple" -- to minimize technical machinery while still engaging you with genuine problems, not vague analogies.
The same author has a book on relativity which looked very good from the first couple of chapters I read.
> has an infinite number of different stupid questions
I think teachers often welcome any question. On coursera for instance teachers are usually available to answer students (via a mail-like interface though, don't expect to chat with them via Skype or anything). There are several courses on general relativity.
Honestly, the best place to do so is an astrophysics undergrad followed by an astrophysics grad program with a good PI. It'll get you there faster and better than any library or self-taught schedule. If you need to choose the self-taught route, look at a course catalog and then research the books they use.
I've not read this yet, but have paged though it a bit at a bookstore, and it looks like it has potential: "The Road to Reality: A Complete Guide to the Laws of the Universe" by Roger Penrose [1].
Can anyone explain why this is getting down voted? As far as I can tell from examining it quite a bit at the bookstore, and from several reviews, it looks like it satisfied the request.
It was interesting to watch, but the presenter presents everything as truth, but aren’t many of his explanations still theories? I’d like to see what other competing theories there are..
I think most of what he said is just an interpretation of what the math of classical black-holes says (basically the Schwartzschild equation). This math is uncontroversial and there is no better theory of a black hole, at least at this level of detail.
If I understand correctly competing theories disagree on more complicated descriptions, those that ask different questions or want to describe more details.
No it doesn’t make sense. That explanation is privledging remote observers frames of reference to “explain” what happens in another frame of reference. That’s a violation of our modern understanding of physics.
It's not "privileging" any thing in the sense that it does not claim one point of view is more valid than the other.
It's just that one of these point of view is ours, and from this point of view, the other point of view does not correspond to any event that will actually happen, like ever.
In other words, the "happening of events" is a relative concept. Just like simultaneity. Not only two events can happen at the same time or not depending on who is observing them, but the very happening of an event can also depend on who is observing (or not observing) it.
That "happening of events" is very close to a definition one can give to the concept of reality. So one might conclude that reality is a relative concept.
I’ve wondered this too but I just had a thought. I ended up asking myself why any black hole is ever bigger than critical mass if this is true.
Take a black hole that’s been slurping matter in and it’s all at the event horizon because as you say, it takes forever to cross. But as that mass builds and builds doesn’t the event horizon move? Take the scene to the limit. Say a black hole never absorbs anything. The amount of mass parked just above the horizon becomes so great that another black hole forms. Now you have two overlapping horizons, which can’t stay that way. What really happens is that the density of mass inside the radius near the black hole is sufficient to be a black hole of a larger diameter and then it just... is.
I think that what this means is that you never get to the event horizon, but the horizon comes to you.
There is so much speculation and misinformation here. One key idea of relativity is that laws of physics remain the same for all uniformly moving reference frames. Objects approaching and entering the event horizon experience time normally. For large black holes, a probe entering past the event horizon will continue to fall towards the singularity and experience time normally for a long time before being destroyed.
To external observers, of course nothing can be observed as falling through the event horizon because not even light can escape past the event horizon. So no light from beyond the horizon of the probe can ever reach the observers. But that doesn't mean things can't go past the EH normally, or that here is some other magic happening like
> I think that what this means is that you never get to the event horizon, but the horizon comes to you.
A black hole is just another object with a very very powerful gravitational field.
GP was talking about the passage of time from the standpoint of the external observer. About time dialation, not about the singularity.
Near the horizon a heartbeat takes eons from the perspective of someone watching you fly into the black hole. Assuming of course that you haven’t already been torn to shreds by the high energy particles and gamma rays traveling perpendicular to your path...
I imagine this as time asymptotically slowing as you approach the singularity so that the singularity is forever stuck at the time of the supernova explosion that gave birth to it. You can never reach it because you and your movements too, slow down the closer you get. Is that correct?
A traveler can reach it all right, but only from his own point of view. From the point of view of almost everyone else in the Universe, including us on Earth, it will never ever reach it. It's like the traveler has jumped into an other universe "located" in an infinitely distant future.
So let's say I fall into a black hole. And then four billion years later, someone else falls into a black hole from the same direction. From an outside perspective at a point nearly infinitely far away in time, we would seem to slow until we are right next to each other at the event horizon, correct? Does this mean everything that falls into a black hole crosses the event horizon (from their perspective) at the same moment, a moment which only exists from the crossers' perspective, as it is always infinitely far away from a perspective outside the event horizon?
> From an outside perspective at a point nearly infinitely far away in time, we would seem to slow until we are right next to each other at the event horizon, correct?
Incorrect. You would still be relativisticly separated and this would be measurable especially at infinity. You would be accelerated by 4 billion more years and would have that much more relativistic acceleration in signals you sent out. This could be monitored and the outside observers would be able to tell the difference. The second person would be more blue shifted compared to you (clock ticking faster).
> Does this mean everything that falls into a black hole crosses the event horizon (from their perspective) at the same moment, a moment which only exists from the crossers' perspective, as it is always infinitely far away from a perspective outside the event horizon?
No, their clocks tick away just like they always did. They cross the event horizon just as they thought they would. They do 'see' the universe's clocks accelerate along though. But they do cross it nonetheless.
Note: By 'see' I mean that the rest of the universe blue shifts to infinity and the Lorentz transformation causes the incoming radiation to be directed to directly in front of you. Only when you cross the event horizon, the incoming radiation from the 'outside' turns into an infinitely energetic (infinite blue shift) beam of radiation coming directly at you. In your reference frame, you vaporize.
This is my BHs are soooo crazy. When we run the numbers, the Quantum Mechanical effects really do affect things. Infinitely small wavelength light isn't something we can really handle, the Plank Length should come into play. But General Relativity don't care. This is why we really need Quantum Gravity, because nothing makes sense.
Yup, from an outside perspective you simply cannot see something cross the event horizon no matter how long you wait, except by detecting the added mass.
If you've not come across Lee Smolin's (speculative) writing on the Fecund Universes theory, you'll probably also love it. Don't have any links as I've only read about it in one of his books, but it's a fascinating idea.
Our own past (say, 1974, for example) is also “separated from us not by a space-like frontier, but a time-like one“. Would you say that the past is not real?
The frontier I was talking about is impassable. It's "infinitely in the future", aka "never". That's a whole different level of frontier, imho.
Granted, there is a bit of tree falling[1] conundrum here, but that does not make the question less interesting (arguably, it even makes it all the more so).
not an physicist, but from the point of view of the thing going on the black whole, time still goes on smoothly. My understanding is that the inside of the black whole becomes like a new universe, effectively cut from the previous one from the perspective of causality
This strikes me as an issue between understanding these events from the point of view of an external observer, and a test particle or person falling into the hole. From the POV of the observer, your statements are largely accurate, for certain types of universes. For the test particle, there is no freeze frame, all proceeds normally.
The key is the Relative in Relativity.
Truthfully, there is no clear answer as to what a black hole has for an interior, or if it really has one. Holographic theorists would argue that the event horizon is the black hole, String theorists would argue that there is an interior, but no singularity. We just don’t know.
Between reality and maths, the maths are the more fundamental thing that is used to explain reality. What you call reality is just some circumstances that your brain got used to.
Even if we'd know the perfect model for particle physics, we still can't apply math straight for simulation. We always prefer an approximation, all real world models are approximations.
> Events behind the event horizon are postponed to "the end of times".
Thank you. To me, this has been obvious since I learned about the time slowing required by GR. Since then, I was unable to understand why people are talking about things "falling into" black holes. (There's an episode of SG-1 where a team is trapped by a black hole, and times keep slowing, but they never realize that this means that team WILL NEVER DIE.)
Reminds me a bit of the old short story The Crystal Spheres. In that universe, once a civilization had attained a certain level of advancement, they intentionally migrated to a black hole to await the arrival of others (the idea being that all advanced civilizations in the universe were separated by so much time, that they suffered from a sort of loneliness).
There's another book whose name is on the tip of my tongue that has a similar premise, except the planet instead of the entire system is encased in a sort of time-dilation field. Naturally, humanity thinks it's an attack.
Wonder if they also explored the possibility that we are in a black hole and the universe structure is recursive.
Maybe the background radiation is the part of the hawking radiation that falls inward. And we perceive it infinite but it’s the dilatated space between the hevent horizon and the naked singularity at the center. And the big bang was the supernova that left the black hole.
I've always liked toying with the idea (in my own head, not an astrophysicist) that space is cyclical, or in your words, recursive.
I love animations where it zooms out farther and farther into space, until it gets to the point where galaxies start to resemble molecular structures.
And then of course The Simpson's did a couch gag where exactly this happens and they zoom out and out into space until it finally zooms out of Homer's own head.
Learning of atoms initially I was given an orbital model, like a solar system, who caused/allowed me to form a similar idea (I think it's quite common?) that solar systems could be atoms in a super-universe. I was so annoyed as a teenager to find that atoms aren't really little solar systems.
When I was ten years old I was meditating and thinking about how each one our consciences are a neuron in gods brain... I will never forget that epiphany as it was before I had studied any philos etc
That's a very interesting idea! Although, if the cosmic background radiation is the hawking radiation falling back toward the event horizon, where is the residual radiation left behind by the supernova (because in the traditional interpretation, CMB the afterglow of the big bang).
We are inside of infinite number of recursive black holes. We are inside infinite number of Homer heads. But we are too small to view whole picture even for level above.
We are in a black hole. The event horizon of the entire universe is on the same order as the observable universe. We are inside the black hole of the universe.
There a germ of the right idea here, but our universe is really more of a black hole backwards. Within the event horizon of a black hole, the singularity is always in the future. Within our universe, we see a singularity in our past.
In short, our universe looks more like a white hole than a black hole.
Couldn’t we just be perceiving time backward? I’m no physicist, but AIUI the equations work either way. As a sci-fi–head, sometimes I just like to speculate—“What if the ‘point’ of the universe is the singularity of the big bang?” (Yeah, pun intended.)
If we’re in a simulation, maybe it’s the final result of the computation. If we’re in a black hole, maybe it’s in the past of the universe within, but in the future of the one without. All good material for a short story, one of these days…
Why not?
You can’t have a black hole without a singularity.
A black hole is formed only when >3 solar masses collapse in a singularity.
Maybe the opposite is theoretically possible, the so called naked singularity in which a singularity is not hidden by an event horizon.
A black hole is a region that light cannot gravitationally escape. It has nothing to do with singularities which often but not always exist inside of a black hole. Small black holes often have singularities just to have enough mass density to capture light. It is not required though — imagine a binary system of two neutron stars that are each just below the mass threshold to collapse into a singularity. It would be a black hole to external observers. Oddly larger black holes require less mass density. More oddly still as you approach the size of the observable universe, the required mass density crosses the cosmic mass density, meaning light emitted from Earth cannot gravitationally escape the universe created by the Big Bang, meaning we exist inside of a black hole as big as the observable universe.
I’m not sure why I’m being so heavily down voted for stating this fact you can find in any cosmology textbook.
> More oddly still as you approach the size of the observable universe, the required mass density crosses the cosmic mass density, meaning light emitted from Earth cannot gravitationally escape the universe created by the Big Bang
This is true...
> meaning we exist inside of a black hole as big as the observable universe.
This is an unwarranted jump. I'd recommend looking at the reference [0] from Sean Carroll.
There is nothing to be learned in that article. He says that the question is nonsense because we don’t know what is outside the observable universe. That’s not a very interesting thought restriction.
Yes-ish. There are two reasons we are living inside an event horizon. First the accelerating expansion of the universe means that light emitted from earth now will never reach most of the observable universe. We can look up and see galaxies that are already beyond our cone of influence. That is the primary effect. But a secondary effect is that the mass of the universe (Big Bang remains) is sufficient even though not dense as to bend light emitted from it back onto a closed trajectory. So if there is any void beyond the cosmos that we see , observers in that void would see us—our entire observable universe—as a black hole.
No, respectfully, not unless you radically redefine what you mean by "black hole".
If by "black hole" you mean an object sourcing a Kerr-Newman-like metric, then you have the problem that the distribution of matter in the distribution we see is not at all like the interior region of such a metric. The metric one can infer from the bulk distribution of the visible matter is best described by a Robertson-Walker metric.
Under time-reversal, galaxies in R-W fall through each observer-centric horizon, break up into dust and gas that in turn combines into denser higher-energy and lower-entropy states.
While this might seem a little like a "white hole" (defined as a time-reversed BH), since under time-reversal a stellar BH has Hawking radiation rush into its vicinity becoming relatively dense at the time of the bust-up of the BH horizon, the difference is on the other side of that point a much lower entropy neutron star or similar body emerges from the high-entropy BH. Even stepping back a bit, macroscopically, galaxies are a lot more structured than either Hawking radiation; nor is Hawking radiation more structured than the cold relic fields crossing into optical detectability of an observer in time-reversed RW or time-reversed asymptotic de Sitter space.
This is important: the densest phase of a universe like ours is (extremely) low entropy, while the densest phase of the black hole is (extremely) high entropy, especially if we measure just at their respective horizons. Entropy here is Boltzmannian: the log relationship between the observed macrostates and microstates that can produce them. (We could even test this directly by comparing the structures in the relic fields with Hawking radiation if we were to find sufficiently low-mass astrophysical black holes; we can however get some details from the ringing modes of BH mergers).
Additionally, we can consider curvature. The extremely low bound on our universe's Weyl curvature allowed by indirect and direct gravitational wave observations (so far) completely precludes a Kerr-Newman-like solution, where Weyl curvature will be very large. Our imaging of ever older galaxies does not reveal them to be spaghettified.
Finally, it's worth noting that while Kerr-Newman and Robertson-Walker can have optical horizons so can many other solutions. For example, there are optical horizons in the Gödel solution but we obviously aren't in a Gödel universe. There are also many solutions which cannot have optical horizons; a torus or Klein bottle universe is clearly different from a Euclidean one, although all of those can be everywhere flat and horizon-free. So I don't think one should promote the presence of optical horizons into the defining characteristic of a metric.
I don't think that's possible. Perhaps the singularity itself will survive a "Big Crunch", but I doubt anything between it and the event horizon will. And without the ability to survive the end of the universe, life is unlikely to emerge inside an orbit within a blackhole. Because of the time dilation, life will evolve there hundreds if not thousands of times slower compared to the rest of the universe. So the lifespan of the universe itself may be insufficient for life to form there.
But if the black hole can survive the big bang, therefore it can exist longer that rest of the universe so it seems to have plenty of time to evolve anything.
Fredric Pohl scifi fans might enjoy Hechee Rendesvous, which is near the end of the Gateway series. Somone finds a way to live inside both Kugelblitzen and black holes I think.
Hrm, Gateway you kinda need to read the first one at least. It lays out his universe of humans discovering an alien transport system with no instructions, and IMHO the novelty of that is the best part. In later books, they stumble around a little before (no big spoiler) getting a bunch of answers about the builders and finding them.
Definitely an almost amusing overlap between different senses of “singularity”.
Such civilizations might be like “simple germs”, their self-sustaining black hole vessels interacting and stirring things together; perhaps the cores of the most “interesting” galaxies are sentient, waiting for one femto-tech civ to arise from a trillion slime-ball planetoids — to make first Contact...
The more general possibility of “intersecting” reality at right angles has been taken up in a few additional works I can think of: in Count to Infinity and Excession.
> There was a second, unrelated paper that suggests that aliens can live inside black holes.
[ ... snip ... ]
> In theory, highly advanced aliens could live on such planets, being unobservable from outside while exploiting the high energies and large time dialtions available.
Wouldn't the aliens just need to wait for the black hole to evaporate in order to escape? We're talking a long time, but still possible? If it was planned just right, perhaps they could make it so they emerge in the new Universe.
Also, isn't the assumption that there is a "big crunch" (also a good name for a cereal if it doesn't exist)? We're not sure whether there is a big crunch or heat death? This still seems plausible whether there is a big crunch or heat death - either way a black hole may be the only way for a civilization to stay alive at "the end".
Being emitted as undifferentiated Hawking radiation, after the black hole has shrunk to microscopic size, and when it finaly ceases to exist doesn't sound like much of a happy ending.
I don’t think it makes physical sense to talk about planets orbiting inside the event horizon. Black holes only shrink due to emitting Hawking radiation from the perspective of our frame of reference at a distance. We can consider the contents of the black hole to be undifferentiated mass/energy. From the perspective of a planet in that situation, that’s kind of meaningless as the process would take infinite subjective time.
If the universe expands in a big bang, collapses in a big crunch, and then expands in another big bang, then it’s not a different universe.
The black hole wouldn’t be older than the universe. The black hole is just older than the last, most recent big bang, and the big bang doesn’t explain anything anymore.
It’s disproven as an event of creation. The big bang just becomes different parlance for turtles all the way down.
what if our universe is expanding because there are other bubbles of universes that pull in all directions?
what if Buddha was right in that there are thousand fold universe systems each with their own unique civilizations?
If there are blackholes older than the universe itself, what if that suggested the universe was cyclical and that we may be in some gazillion-th iteration?
what if big bang was just a universe eventually being consumed by a monstrous blackhole that collapses itself?
What about the strange UFO occurences that pentagon acknowledged, are we getting visits from aliens within our universe or from another universe? Have they figured out how to travel between multi-verse, that is if you believe in it?
No proof, no way to find these answers but it really makes me wonder, especially when particles behave strangely like being coupled regardless of distance...but how could Buddha have known that blackholes existed?
>What about the strange UFO occurences that pentagon acknowledged, are we getting visits from aliens within our universe or from another universe?
The Pentagon has never acknowledged the existence of extraterrestrial craft, or of UFOs as being anything but hoaxes and misidentified, mundane phenomena.
>but how could Buddha have known that blackholes existed?
He didn't, any more than the Norse knew that wormholes existed when describing the Bifrost bridge or the World Tree. Reading modern scientific meaning into ancient religious ideas is a common way to attempt to validate religion, but doing so does a disservice both to the religion and to science.
The Buddha may have had many insights, but none of them involved the relationship between spacetime and gravity.
The US government hasn't acknowledge the existence of aliens, but in 2016 the CIA admitted to funding programs because they suspected the existence of aliens. It is very hard to find a good source to link though due to terrible SNR when you google "cia aliens"
The government also studied remote viewing, and the Air Force had its own UFO project called Operation Blue Book, and of course there was MKULTRA looking into mind control, but just because something is studied doesn't mean it exists, or gives practical results.
Sundevil (remote viewing) and Blue Book (UFOs) were '60s projects. AATIP and the UAP footage has been confirmed by the Pentagon itself, and even Raytheon's official site talks up the use of their ATFLIRs in recording the footage:
Like Newton's Principia (from 1684) looks like Norse mythology? It is still a valid model except for a few minor errors, and almost halfway to 1000 years. Better yet might be Greek mathematics, e.g. Euclid's Elements which is more than 2300 years old now. The language is different of course, but the content itself reads a lot like a good proof-oriented geometry textbook.
Yes there are. Answering such questions is why physicists are searching so hard for a unified micro/macro theory. Black holes are both quantum mechanical (at their singularities) and relativistic.
> how could Buddha have known that blackholes existed?
They point out that as you approach a black hole, your proper time is advancing slower and slower when compared to an observer located far away from the black hole (like us, on Earth). That means that from the point of view of this far observer, nothing happens inside a black hole until an infinite amount of time. In other words, what is inside a black hole is separated from us not by a space-like frontier, but a time-like one. Events behind the event horizon are postponed to "the end of times".
Does it even make sense to say that those events exist, then? Are they any more real than fairy tales or mathematical equations? If not, we can make any speculation we want, including the existence of alien civilizations.
1. https://www.youtube.com/watch?v=vNaEBbFbvcY
But the stuff that falls in is not purely described by its mass. It also has angular momentum, electrical charge, maybe a few other quantities such as charm and strangeness (speculating here). So the black hole must have a net angular momentum, electrical charge, charm, and strangeness. Electrical charge could be measured by holding an electrometer next to the black hole. And so forth.
So far I've described 5 quantities that "describe" a black hole. So my question is: How many more are there? What's the total number of numbers that accompany a particle as it gets sucked in?
Suppose it's a small number, like 11. Does that place an upper limit on the information content of a black hole, and hence the possibility of describing a "civilization" inside it?
The singularity is a prediction of GTR, but GTR breaks down there too. This is why black holes are so exciting in the context of new theories which are complementary to GTR!
It even preserves relative motion ! If A and B are both falling towards the event horizon, their relative movement doesn't change : by that I mean that if it was 2 moons, and someone launched a rocket from A towards B, it would take, say 10 hours to get to B. When both fall towards the black hole that time can still lengthen from the perspective of someone standing on A or B. It can shorten. It can stay the same. Depends on how it was changing before they started falling in.
So it could simply be that relativity stretches the space time near the event horizon enough for everything to fit in there, like that tent in Harry Potter. It looks weird from the outside, but if you're falling into the black hole it's the opposite: everything outside of that (very, very large) space near the event horizon is what looks weird. But if the black hole is big enough, it looks weird, but ... not very much.
The entirely weird thing is, you can choose initial conditions where all distances lengthen proportionally, in fact that's the more common scenario. So for all objects that are falling into the black hole, and objects not falling into it, all distances lengthen. Make the black hole big enough and the difference between things falling into it and things orbiting it or even moving away are very very small indeed.
And now you look at our universe, and that's exactly what you see. The value of the cosmological constant (ie. the universe is expanding, but ever slower and it will never actually stop expanding) can be explained by the assumption that we're falling into a very, very large black hole.
There is a TON of work done on BHs and the formulations to try to bend our knowledge of QM and 'classical' physics to them. If you really want to get into the nitty gritty of it start with this [1]. It's a HARD road, but the proofs and knowledge in the field is mindblowingly cool. Go for it!
[0]https://en.wikipedia.org/wiki/Relativistic_electromagnetism
[1]https://www.amazon.com/Classical-Electrodynamics-Third-David...
I've still got my copy of Jackson from grad school. ;-)
You'll probably find MTW a lot more approachable after working through a few less beastly texts.
That's exactly the right number to describe the macroscopic state of a no-hair black hole (defined as the horizon as seen from the outside) using a set of spatial coordinates: mass, charge, angular momentum for three spacelike axes, linear momentum for three spacelike axes, and centre-of-mass position for three spacelike points. Here I have already done a foliation of spacetime into a spatial slice with a constant timelike coordinate; it'd be normal to use coordinates in which the centre of mass is always at the origin, which fixes the last six components at zero.
On the other hand, if we omit the spacetime foliation, we are obliged to use tensor quantities for these variables; moreover, if the spacetime region we consider is not "sufficiently small", then no-hair looks much more conjectural. Foliating this region, we would expect at least the charge no-hair number not to change from one slice to another. For a black hole with a highly ionized accretion disk with nuclei or electrons liable to cross the event horizon at any time, this is a tough ask (and the subject of research in numerical relativity).
> charm and strangeness
The larger the black hole, the flatter the spacetime just outside the event horizon; astrophysical black holes' immediate neighbourhood is too flat to break quark confinement, so a surplus of colour charge seems unlikely. Conversely, a black hole small enough that tidal effects are very strong just outside the horizon is likely to be evaporating so violently that gravitational effects on hadrons just outside the horizon seems (to me, anyway) less interesting than scattering interactions.
> What's the total number of numbers that accompany a particle as it gets sucked in?
Good question. This is an area of research.
Any answer raises a second question: how does "balding" work?
Even fully classically we have this problem: if we drop a thin uniform spherical shell of neutral matter of mass M into a Schwarzschild black hole (so there's no rotation or charge or quantum-anything to consider) how do we distinguish that black hole from an identical setup except we drop in two concentric shells of 1/2 M each?
If in some future region of spacetime when the shells are inside the BH (black hole) we can distinguish between BH with one shell vs BH with two shells, then no-hair is wrong. If we cannot distinguish, then classical information is lost.
This is black hole thermodynamics because we have a relationship between macrostates (the no-hair values) and microstates (the set of values that migrated from outside the BH to inside the BH), and we can define entropy in a Boltzmannian way using that relationship. If no-hair is accurate such that we can throw in an huge number of shells each with a tiny fraction of the mass of our single-shell example, then that black hole's entropy is enormous.
The quantum picture is in some ways "just" a complication of this fully classical information loss problem. If we can throw in whole molecules / bits-of-dust, atoms and ions of various masses, free electrons, photons, neutrinos, and so forth and still see a "no-hair" set of macroscopic variables, then a reasonable-size BH's entropy is enormous, and it gets much more enormous if we only increase the BH mass while keeping the other no-hair values always zero (and letting the others vary does not help much).
If in our universe we find a black hole which we can comfortably describe with a tiny number of variables (e.g. a no-hair black hole), we should expect that it will have lots of hidden microstates thanks to things having infallen (and indeed, for astrophysical black holes, lots of particles from the stellar remnant). Did these bald away in the past? Or, if black holes evaporate, will these hidden microstates be revealed during that process?
> what's inside
Good question. There's a wide variety of guesses by theoreticians.
As we increase our number of observations of BH-BH, BH-NS, and NS-NS (NS for neutron star) mergers where we get decent gravitational wave signals, we can exclude many possible answers to these problems.
Finally,
> an upper limit on the information content of a black hole
No-hair black holes' event horizons are determined by the no-hair variables alone; their entropy limit is related to the surface area of the horizon.
> a "civilization" inside it
We don't know without a full answer to "what's inside". The "Eldar" idea is that for an extremely massive black hole with an improbably large charge (how does one prevent such an object from drawing in matter of the opposite charge?) there may be stable orbits in an interior region. I think even that is a really big stretch.
So how do black holes gain any mass then?
Well, from our point of view all incoming mass gets stuck very close to the event horizon. That's not too surprising considering the amount of information of a black hole is proportional to its surface area. So from a far away observer a black hole is more like a sticky sphere than an hollow ball.
At that point we probably can't tell more without diving into the math, and it's beyond my abilities. Yet the gross idea does not seem absurd to me.
It is also important to distinguish between the implications of a model black hole formed from the uniform collapse of a perfectly spherical dust cloud, from infinity, in an otherwise empty universe, with no charge or angular momentum, from what happens in nature.
So in that case, you wouldn't be seeing the "future of the universe" just the "future" (I guess) of the space around the black hole.
I kind of see blackholes in space as vortexes in a lake. Anything that gets "trapped" in the vortex moves much faster, including the water (space-time) itself. It doesn't impact the rest of the lake, except for the things that are on a collision course with the vortex, and then get swallowed by it (and spit back out?).
So I had the idea that smaller black holes are at the center of the sun, the earth and so on, being the principle source of gravity and the "movement" that we see is just us falling into different black holes at the same time, which are also falling into each other. So micro black holes must be at the center of massive particles too. The world line of a photon on the other hand is just the intersection of two event horizons as they grow, so you get a wave model. And that's why you have entanglement: circles have two intersections, so if your model is two dimensional, you get two entanglements. But you can have vastly more complicated geometries and thus assembles of entangled particles.
I don't know the "standard model" well enough to take the analogy any further, not to mention string theory and all that jazz.
>So micro black holes must be at the center of massive particles too.
If an object has a schwarzschild radius smaller than its own radius, then it isn't a black hole. That literally describes all non-black-hole objects with mass. That's just the standard manifestation of gravity.
>The world line of a photon on the other hand is just the intersection of two event horizons as they grow, so you get a wave model. And that's why you have entanglement: circles have two intersections, so if your model is two dimensional, you get two entanglements.
Is there any connection here besides that an event horizon and the sum of all possible paths of a photon in a given amount of time are both spheres?
If there were a micro-black-hole inside of a particle, its event horizon would have to be within the particle, or else the particle would just be indistinguishable from a black hole. The particle wouldn't be like a normal particle with an invisible spherical event horizon surrounding it and affecting its interactions.
I think you meant to say ether.
This is not an analogy as much as an example of an external-vs-internal observer problem. When you close the (opaque, insulated) door of your fridge, observers inside will see the (filament of the incandescent) light significantly dim, and if the door stays closed long enough, will see the light thermalize with rest of the internal volume. Someone standing outside the fridge might not even see the initial dimming; indeed, that observer may only ever see the light as "on" (rather than "heating from cold" or "cooling from hot").
[1] We could talk about naked singularities a bit: this usually means that there is at least one outside-the-black-hole observer for which the shape of the horizon is such that the centre of mass-energy of the BH is outside the horizon, rather than an observer for which there is no horizon at all. However, even these scantily clad BHs don't arise in realistic universes described by General Relativity. Fully naked singularites (where at least one observer exists which does not see any horizon at all) require an alternative theory of gravitation, or conditions extremely unlike those anywhere in our universe.
[2] Consider the observation of a supermassive black hole at the edge of the observable universe. From our view here around Earth, we see a race between a very bright star about to cross the black hole's event horizon and the black hole about to cross our Hubble horizon. Observatory A sees the star vanishing behind the horizon just in time; Observatory B sees the BH cross out of observability before the star vanishes behind the BH horizon. A and B have (very slightly) different Hubble horizons focused on them [3], and also with a (n also slightly) different radial distance to the BH horizon. "B" can never directly see the same coincidence of events that "A" sees; should "B" deny the infalling?
[3] Maybe this is illustrative of observer-centred observables? Glories (an optical phenomenon similar to rainbows) are so observer-specific that you and your handheld camera will have different ones (and each of your eyes will have different ones). As noted in the "From the air" subsection, we can tell what seat a photographer of a glory from a plane must have been sitting in. https://www.atoptics.co.uk/droplets/gloim1.htm Likewise, we can determine the location in spacetime of an observer of a star-into-black-hole event from that observer's detailed description.
My immediate, naive instinct is that by crossing this limit, the frame dragging effect, would be extremely powerful, to the point where it might increase the radius where Hawking radiation is formed/emitted and increase the rate of Hawking radiation to avoid passing the limit. A sort of self limiting process to prevent breaking the speed limit of c.
But in the time it took to write this out, I remembered angular momentum and realised that notwithstanding the additional angular momentum of the infalling mass, the conservation of angular momentum would just make the event horizon slower as it expands. Which makes the superluminal event horizon unlikely in my mind.
The biggest issue, aside from the model, is that time dilation is something which only matters when two observers “compare clocks.” Neither observer alone ever experiences a difference. The crew of a 99.9% lightspeed ship doesn’t experience time dilation... until they return home. It makes no sense to talk about the effects of time dilation from the point of view of a one-way trip to the event horizon.
That's not true. They see the universe around them moving much faster.
Time dilation has nothing to do with "returning home".
Common sense dictates that the probe and the sample would require equal fuel to return to earth; but to an observer riding this cigar-rock, why would the universe cut our probe some slack if we changed its kinetic energy rather than the observer?
The biggest issue, aside from the model, is that time dilation is something which only matters when two observers “compare clocks.” Neither observer alone ever experiences a difference. The crew of a 99.9% lightspeed ship doesn’t experience time dilation... until they return home. It makes no sense to talk about the effects of time dilation from the point of view of a one-way trip to the event horizon.
That has to do with the experience of their frame of reference. Time does appear to “slow down” for them, rather everything else will seem to “speed up.” You can infer the difference, but you can’t sctually communicate that or compare with anyone else until you decelerate. In the extreme case of a gravitational event horizon, there will be no ability to ever communicate again. The fact that external observers will see you infinitely redshifted doesn’t imply anything about your experience of subjectively falling past the horizon. Both are valid frames of reference, but ultimately will develop spacelike separation which prohibits further communication.
As it relates to the issue st hand, you can’t make accurate statements about mass never passing through the EH based on observations from a distant from of reference.
So, if you don't sense anything, you don't sense time dilation either?
This is slightly more complicated. First of all, you haven't given a frame of reference. If you claim someone were moving at 0.99c then you have already set the frame of reference. And they would have to gain near infinite mass and would die. You seem to assume a restricted frame of reference though, inside the spaceship. So, a point of reference inside the spaceship would see light moving with c inside the spaceship. And would assume his own point of reference as the origin of the inertial frame of reference. So baring any outside measurement, how do you know the spaceship is moving with 0.99c and in which frame of reference?
Recently, I worked out the ISS orbit using the Schwartzchild metric as an approximation of Earth. It's cool to see the orbital period pop out and agree with real life! It's then just a small step to calculate the time dilation experienced by ISS astronauts.
The answer is kinda easy if I can make up my own intrinsic definition. The mass is the mass of the stuff around the black hole. A black hole is a singular point, it can't have mass, don't be silly.
I have heard "the theoretical minimum" suggested in the past, but I haven't heard from anyone who has actually used it to go from zero to say, general relativity, only people who think it sounds good. It also seems like a steep investment -- I just want to have a decent enough grasp to separate sense from nonsense and hold a picture in my mind.
One related thing I have wished existed was a good place to ask questions about and discuss topics like this while learning them, but it seems like in theoretical physics, everyone (including myself) has an infinite number of different stupid questions and misconceptions, to the point that we drown each other out, and answers to our questions often have little cross-applicability.
So, would love ideas for either!
You say you haven't heard from anyone who has used it to go from zero to say, general relativity. You might want to look into the adult alumni of the class.
EDIT: You might also find another HN post interesting, it's currently at the top of HN classic: "Learning classical mechanics through Haskell", here: https://news.ycombinator.com/item?id=16453192
(And a direct link to "Classical Mechanics via Scheme", https://mitpress.mit.edu/sites/default/files/titles/content/...)
See: http://theoreticalminimum.com/courses
Nobody has ever discovered a way to truthfully map physics concepts into English sentences that you can understand without having to have explored the entire definition tree.
This route provides a gentle, productive ramp as you make sure you really want to dive deep.
I've done that and, ideally, I'd like to now buy some college physics books. However, I may have to delay this next step for quite a while due to some other higher priority goals... We will see if I get back to it. There is too much interesting shit and important things to do in the world, heh.
It's a little bit like The Little Schemer in that it often asks you a question and expects you to try to answer it before turning the page (or turning it upside down). The problems tend to go like "if you did this, would that (A) increase (B) decrease (C) stay the same" or "(A) increase by one (B) double (C) quadruple" -- to minimize technical machinery while still engaging you with genuine problems, not vague analogies.
The same author has a book on relativity which looked very good from the first couple of chapters I read.
I think teachers often welcome any question. On coursera for instance teachers are usually available to answer students (via a mail-like interface though, don't expect to chat with them via Skype or anything). There are several courses on general relativity.
https://aeon.co/ideas/what-i-learned-as-a-hired-consultant-f...
It's incomplete but there's some good stuff there.
For Example: http://astro.cornell.edu/undergraduate-studies.html
[1] https://www.amazon.com/Road-Reality-Complete-Guide-Universe/...
Is there something about it that I have missed?
This is mind blowing...shatters my misconception of blackholes....is there any more books on this?
If I understand correctly competing theories disagree on more complicated descriptions, those that ask different questions or want to describe more details.
It's just that one of these point of view is ours, and from this point of view, the other point of view does not correspond to any event that will actually happen, like ever.
In other words, the "happening of events" is a relative concept. Just like simultaneity. Not only two events can happen at the same time or not depending on who is observing them, but the very happening of an event can also depend on who is observing (or not observing) it.
That "happening of events" is very close to a definition one can give to the concept of reality. So one might conclude that reality is a relative concept.
Take a black hole that’s been slurping matter in and it’s all at the event horizon because as you say, it takes forever to cross. But as that mass builds and builds doesn’t the event horizon move? Take the scene to the limit. Say a black hole never absorbs anything. The amount of mass parked just above the horizon becomes so great that another black hole forms. Now you have two overlapping horizons, which can’t stay that way. What really happens is that the density of mass inside the radius near the black hole is sufficient to be a black hole of a larger diameter and then it just... is.
I think that what this means is that you never get to the event horizon, but the horizon comes to you.
To external observers, of course nothing can be observed as falling through the event horizon because not even light can escape past the event horizon. So no light from beyond the horizon of the probe can ever reach the observers. But that doesn't mean things can't go past the EH normally, or that here is some other magic happening like
> I think that what this means is that you never get to the event horizon, but the horizon comes to you.
A black hole is just another object with a very very powerful gravitational field.
Near the horizon a heartbeat takes eons from the perspective of someone watching you fly into the black hole. Assuming of course that you haven’t already been torn to shreds by the high energy particles and gamma rays traveling perpendicular to your path...
Incorrect. You would still be relativisticly separated and this would be measurable especially at infinity. You would be accelerated by 4 billion more years and would have that much more relativistic acceleration in signals you sent out. This could be monitored and the outside observers would be able to tell the difference. The second person would be more blue shifted compared to you (clock ticking faster).
> Does this mean everything that falls into a black hole crosses the event horizon (from their perspective) at the same moment, a moment which only exists from the crossers' perspective, as it is always infinitely far away from a perspective outside the event horizon?
No, their clocks tick away just like they always did. They cross the event horizon just as they thought they would. They do 'see' the universe's clocks accelerate along though. But they do cross it nonetheless.
Note: By 'see' I mean that the rest of the universe blue shifts to infinity and the Lorentz transformation causes the incoming radiation to be directed to directly in front of you. Only when you cross the event horizon, the incoming radiation from the 'outside' turns into an infinitely energetic (infinite blue shift) beam of radiation coming directly at you. In your reference frame, you vaporize.
This is my BHs are soooo crazy. When we run the numbers, the Quantum Mechanical effects really do affect things. Infinitely small wavelength light isn't something we can really handle, the Plank Length should come into play. But General Relativity don't care. This is why we really need Quantum Gravity, because nothing makes sense.
I also really need one of these:
https://store.dftba.com/collections/all/products/heat-death-...
Granted, there is a bit of tree falling[1] conundrum here, but that does not make the question less interesting (arguably, it even makes it all the more so).
1. https://en.wikipedia.org/wiki/If_a_tree_falls_in_a_forest
The key is the Relative in Relativity.
Truthfully, there is no clear answer as to what a black hole has for an interior, or if it really has one. Holographic theorists would argue that the event horizon is the black hole, String theorists would argue that there is an interior, but no singularity. We just don’t know.
Thank you. To me, this has been obvious since I learned about the time slowing required by GR. Since then, I was unable to understand why people are talking about things "falling into" black holes. (There's an episode of SG-1 where a team is trapped by a black hole, and times keep slowing, but they never realize that this means that team WILL NEVER DIE.)
https://en.wikipedia.org/wiki/The_Crystal_Spheres
Really good book
A similar theme is in "Pushing Ice" by Alastair Reynolds.
Someone has built machines that capture beings and then use near-light speed travel to align civilization in time.
>They would also be brightly illuminated by the central singularity and by photons trapped in the same orbit.
That sounds creepily like a typical description of heaven. Timeless, ageless, godlike beings all in a region of intense uniform brightness.
http://www.gregegan.net/PLANCK/Complete/Planck.html
Maybe the background radiation is the part of the hawking radiation that falls inward. And we perceive it infinite but it’s the dilatated space between the hevent horizon and the naked singularity at the center. And the big bang was the supernova that left the black hole.
Trippinng!
I love animations where it zooms out farther and farther into space, until it gets to the point where galaxies start to resemble molecular structures.
And then of course The Simpson's did a couch gag where exactly this happens and they zoom out and out into space until it finally zooms out of Homer's own head.
In short, our universe looks more like a white hole than a black hole.
If we’re in a simulation, maybe it’s the final result of the computation. If we’re in a black hole, maybe it’s in the past of the universe within, but in the future of the one without. All good material for a short story, one of these days…
I’m not sure why I’m being so heavily down voted for stating this fact you can find in any cosmology textbook.
This is true...
> meaning we exist inside of a black hole as big as the observable universe.
This is an unwarranted jump. I'd recommend looking at the reference [0] from Sean Carroll.
[0] http://www.preposterousuniverse.com/blog/2010/04/28/the-univ...
https://en.wikipedia.org/wiki/Event_horizon#Cosmic_event_hor...
If by "black hole" you mean an object sourcing a Kerr-Newman-like metric, then you have the problem that the distribution of matter in the distribution we see is not at all like the interior region of such a metric. The metric one can infer from the bulk distribution of the visible matter is best described by a Robertson-Walker metric.
Under time-reversal, galaxies in R-W fall through each observer-centric horizon, break up into dust and gas that in turn combines into denser higher-energy and lower-entropy states.
While this might seem a little like a "white hole" (defined as a time-reversed BH), since under time-reversal a stellar BH has Hawking radiation rush into its vicinity becoming relatively dense at the time of the bust-up of the BH horizon, the difference is on the other side of that point a much lower entropy neutron star or similar body emerges from the high-entropy BH. Even stepping back a bit, macroscopically, galaxies are a lot more structured than either Hawking radiation; nor is Hawking radiation more structured than the cold relic fields crossing into optical detectability of an observer in time-reversed RW or time-reversed asymptotic de Sitter space.
This is important: the densest phase of a universe like ours is (extremely) low entropy, while the densest phase of the black hole is (extremely) high entropy, especially if we measure just at their respective horizons. Entropy here is Boltzmannian: the log relationship between the observed macrostates and microstates that can produce them. (We could even test this directly by comparing the structures in the relic fields with Hawking radiation if we were to find sufficiently low-mass astrophysical black holes; we can however get some details from the ringing modes of BH mergers).
Additionally, we can consider curvature. The extremely low bound on our universe's Weyl curvature allowed by indirect and direct gravitational wave observations (so far) completely precludes a Kerr-Newman-like solution, where Weyl curvature will be very large. Our imaging of ever older galaxies does not reveal them to be spaghettified.
Finally, it's worth noting that while Kerr-Newman and Robertson-Walker can have optical horizons so can many other solutions. For example, there are optical horizons in the Gödel solution but we obviously aren't in a Gödel universe. There are also many solutions which cannot have optical horizons; a torus or Klein bottle universe is clearly different from a Euclidean one, although all of those can be everywhere flat and horizon-free. So I don't think one should promote the presence of optical horizons into the defining characteristic of a metric.
> We are in a black hole.
This is straightforwardly wrong. Solve the null geodesics in the near-horizon for any observer you care to conjecture, and you'll see.
Could it be that these type III civilizations are simply consuming so much energy that that in itself becomes the black holes?
The KARDASHEV Scale explained. https://www.youtube.com/watch?v=9k-Kuc9esDI
Such civilizations might be like “simple germs”, their self-sustaining black hole vessels interacting and stirring things together; perhaps the cores of the most “interesting” galaxies are sentient, waiting for one femto-tech civ to arise from a trillion slime-ball planetoids — to make first Contact...
The more general possibility of “intersecting” reality at right angles has been taken up in a few additional works I can think of: in Count to Infinity and Excession.
[ ... snip ... ]
> In theory, highly advanced aliens could live on such planets, being unobservable from outside while exploiting the high energies and large time dialtions available.
reminds me of Frederick Pohl's Heechee Saga.
Also, isn't the assumption that there is a "big crunch" (also a good name for a cereal if it doesn't exist)? We're not sure whether there is a big crunch or heat death? This still seems plausible whether there is a big crunch or heat death - either way a black hole may be the only way for a civilization to stay alive at "the end".
The black hole wouldn’t be older than the universe. The black hole is just older than the last, most recent big bang, and the big bang doesn’t explain anything anymore.
It’s disproven as an event of creation. The big bang just becomes different parlance for turtles all the way down.
what if Buddha was right in that there are thousand fold universe systems each with their own unique civilizations?
If there are blackholes older than the universe itself, what if that suggested the universe was cyclical and that we may be in some gazillion-th iteration?
what if big bang was just a universe eventually being consumed by a monstrous blackhole that collapses itself?
What about the strange UFO occurences that pentagon acknowledged, are we getting visits from aliens within our universe or from another universe? Have they figured out how to travel between multi-verse, that is if you believe in it?
No proof, no way to find these answers but it really makes me wonder, especially when particles behave strangely like being coupled regardless of distance...but how could Buddha have known that blackholes existed?
https://www.reddit.com/r/Buddhism/comments/6o8zmg/the_buddha...
Anyways, just some things to ponder and gawk about...
The Pentagon has never acknowledged the existence of extraterrestrial craft, or of UFOs as being anything but hoaxes and misidentified, mundane phenomena.
>but how could Buddha have known that blackholes existed?
He didn't, any more than the Norse knew that wormholes existed when describing the Bifrost bridge or the World Tree. Reading modern scientific meaning into ancient religious ideas is a common way to attempt to validate religion, but doing so does a disservice both to the religion and to science.
The Buddha may have had many insights, but none of them involved the relationship between spacetime and gravity.
https://www.raytheon.com/news/feature/uap_atflir
We don't know if the footage is aliens, but we do know that the footage is real.
Yes there are. Answering such questions is why physicists are searching so hard for a unified micro/macro theory. Black holes are both quantum mechanical (at their singularities) and relativistic.
> how could Buddha have known that blackholes existed?
He didn’t.
Doesn't look like a description of black hole, honestly.